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In providing this basic function, it is necessary to consider such important aspects as human safety, as any local interaction and trafficking by humans in conducting their ordinary duties whilst in close proximity to these LP and Earthing systems, has a risk, whilst the segregation of internal cabling systems and sensitive equipment from the elevated electro magnetic fields generated by down-conductors and earthing systems during lightning strikes, is also an important consideration.
It is important to ensure that any lightning Protection system down-conductor is connected to a low R (resistance) and low Z (characteristic impedance) earthing system, one which is designed to dissipate the lightning current into the ground as quickly as possible, whilst minimizing ground potential rise, and maximizing the potential fall-off, away from the currents earth injection point.
The Australian Standard AS/NZS 1768-2007 “Lightning Protection” provides useful guidance in this respect, and it is recommended that whilst this is not a “mandatory” standard, if the recommendations and design considerations are implemented in their entirety, the greatest level of acceptable protection can be afforded to both equipment and personnel.
The generic Lightning Protection design methodology that is endorsed in the AS1768 Standard is based on the adoption of the Rolling Sphere Methodology (RSM), which is used in determining zones of protection. In this methodology, an imaginary sphere of specified radius (based on design protection levels “see below”) is theoretically brought up to, and rolled over the total structure to be protected. All sections of the structure that the sphere touches are considered to be exposed to direct lightning strokes, and would need to be protected by the placement of air terminals (lightning rods).
The illustration below visually demonstrates the concept behind Rolling Sphere Method (RSM). Note that the 45 mtr sphere sits atop the air rods, thereby providing an air gap between the sphere and the structure.
In general, air terminals need to be installed in such a manner so that the rolling sphere only touches their upper interception surfaces, and when/ if the sphere touches or breaches the outer fabric of the structure, then additional air terminals should be installed at those points. The values of the rolling sphere radius are based on four protection levels:
AS1768-2007 provides the relevant radius of the rolling sphere for each protection level, together with the corresponding minimum lightning current (Imin) that will be intercepted.
Rolling Sphere Radius for Each Protection Level
I min-kA
For normal applications Protection Level 3 is recommended using the 45 Mtr rolling sphere. Where munitions, fireworks or explosives are concerned Protection Level 1 is required using a 20 mtr rolling sphere.
Conventional Design using 45Mtr RSM
Conventional Lightning Protection Systems should be designed and installed during a structures construction as this type of Lightning Protection system can be difficult to retrofit to existing buildings, and would require down conductors to be installed vertically on the external walls every 20 mtrs. These conductors also require to be clipped every meter or so, hence the LP system be visually unsightly and can detract from the aesthetics of the building markedly.
During construction, structural rebar within the concrete may be used to carry lightning current to ground where there is a continuity of the rebar from roof to ground. Where this continuity is not assured, the use of dedicated down conductors is recommended.
These systems are very labor and component intensive, where the install costs can be considerable as an overall percentage of the systems cost.
Alternatively there is another Lightning Protection system which is commonly used, although is outside of the scope of the AS1768 Lightning Protection standard, the Non Conventional “System 3000” Lightning Protection System.
Non-Conventional Systems 3000 LPS
The System 3000 is not covered in and is outside the scope of the current AS/NZS 1768-2007 Lightning Protection standard, but are proven to be effective and are simple and easy to retrofit to existing buildings. The system generally require very few down conductors, (the down conductor is a special design coaxial cable) which are relatively flexible and can be route current away from any sensitive areas. The condustor can be taken down areas of the structure which do not distract from the aesthetics. A single LPS would require a single coaxial down-conductor.
As installation would generally take a day or so, compared to a conventional type system, significant cost savings can be realized by the use of System 3000
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